1. Field of the Invention
This invention relates to processing of a radiation image signal and more particularly to a method of and apparatus for processing a radiation image in which frequency response processing can be selectively applied to only a pattern of a tissue out of patterns of a plurality of tissues falling within similar spatial frequency regions.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order for the desired image density to be obtained, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal (image signal) to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT.
In the radiation image recording system using an X-ray film or the stimulable phosphor sheet, the dose of radiation to which the object is exposed should be as small as possible especially when the object is a human body. However as the dose of radiation reduces, the graininess of the image deteriorates due to influence of quantum noise and the reproduced image becomes rough.
In order to overcome such a problem, there have been proposed various methods of improving the graininess of the image on the basis of the image signal taking into account other image qualities such as the sharpness of the image. (See, for instance, Japanese Unexamined Patent Publication No. 64(1989)-23676 and U.S. Pat. No. 5,051,902. In these methods, a specific spatial frequency component of a radiation image is emphasized relative to the other components in order to improve the total image quality.
However, when a single radiation image contains therein patterns of a plurality of tissues whose spatial frequency components overlap each other, there is a case where the frequency response processing contributes to improvement in the image quality of the pattern of one of the tissues but adversely affects the image quality of other tissues. For example, when, in a radiation image of the chest of a human body which is constituted of soft tissues such as the lungs and the heart and bones such as the ribs, the pattern of the soft tissues is to be reproduced as a visible image, frequency response emphasizing processing for improving ease of diagnosis through the pattern of the soft tissues emphasizes both the patterns of the soft tissues and the bones, and in the visible image reproduced on the basis of the processed image signal, the pattern of the bones near the soft tissues is too conspicuous though the image quality of the pattern of the soft tissues itself is improved.